Edge-lit LED backlight module has become the mainstream design of the current backlight module because of its advantages such as energy conservation, environmental protection, low power consumption and ultrathin. With maturity of the LED technology, the backlight module design using the high-power less-number LED has further become the irresistible general trend. However, the surface temperature of LED of this design rises up quickly, and therefore higher requirements are put forward for the heat resistance of the related optical members. In the prior art, the reflection sheet of the edge-lit LED backlight module is usually close to the LED, and thus very easy to expand due to the high temperature at the light-entering side of the LED, so that it produces wavy warp in a heated state, which affects the optical quality of the entire edge-lit LED backlight module.